First let's have a look at some knee walking:
Historically, knee walking has been done in Japan for centuries in both civilian and military life. In civilian life in old Japan most of the activities done inside the home or a building were done on the knees such as cooking, eating or even discussing business. In addition to this, most typical Japanese buildings were never very tall because natural resources such as wood were very scarce and costly, so standing up to perform tasks was not an option. Knee-walking became a common practice in family life.
During Feudal times, the Samurai would be expected to sit and walk around on their knees while in the presence of a daimyo (feudal lord). This was also a position in which one received guests, not all of whom were always trustworthy, so in theory, keeping everyone low to the ground made it more difficult for anyone to attack the daimyo. However, samurai still had to function as warriors and bodyguards and so trained to fight, if necessary, from the iaigoshi position. Iaigoshi is similar to the seiza (kneeling) position but the balls of the feet remain on the ground so that you are always in a position to move quickly, either by walking on the knees or by leaping to the feet.
It is fairly easy to see why, in the art of the Japanese sword, shikko is still practised along with learning sword techniques from the iaigoshi position. These traditional samurai techniques are integral to the art. However, why is shikko practised so widely in aikido, which is a modern budo art?
Aikido is a blend of (mainly) grappling arts (Daitō-ryū aiki-jūjutsu, Tenjin Shin'yō-ryū Gotōha Yagyū Shingan-ryū and judo) Though Daitō-ryū is the primary technical influence on aikido, it also derives much of its technical structure from the art of swordsmanship (kenjutsu). Many of the strikes of aikido are often said to resemble cuts from a sword or other grasped object, which indicates its origins in techniques intended for armed combat. Other techniques, which appear to explicitly be punches are also practiced as thrusts with a knife or sword.
This influence of the sword in aikido extends to learning techniques from the iaigoshi or seiza position (suwari waza - both uke and nage seated; hanmi handachi - uke standing, nage seated) in much the same way as it is practised in kenjutsu and thus the need to learn shikko becomes more apparent.
Training to do shikko has many positive physical benefits. It increases strength and flexibility in the legs and hips. The rotational movement required to walk in shikko is particularly good for getting one used to engaging the hips properly when moving and is very important for developing a strong awareness of one's center of gravity (hara or lower dantian).
So how do you do it?
1. From iaigoshi position: drop the right knee to the floor
2. without raising the hips, step forward to assume a left-leg leading iaigoshi.
3. repeat the movement to continue moving forward in a straight line.
While moving in shikko, keep the balls of the feet in a straight line and avoid raising and lowering the body. Ref: Bokken - art of the Japanese Sword, p.58, by David Lowry.
Here's another description of how to knee walk from David Harvey: http://www.gedanate.com/martialarts/japanese/aikido/shikko-knee-walking/
"Imagine your ankles are tied together with a set of elastic bungeeTo finish - a look at the application of shikko in suwari waza (the overhead filming of suwari waza randori is particularly impressive at the beginning of this video)
You are kneeling. Then you lift up your right knee and place the right
foot flat to the floor.
The imaginary ‘elastic’ pulls both your heels together, so your left heel swivels across to touch your right heel. The heels are together again.
Using your hip, and leaving your feet where they are, allow your right knee to kneel down again. You have just moved forward about 19 inches (50 cms).
Now raise the left knee and bring the left leg forward so the foot is flat on the ground and the left knee is raised. (Your ankles are apart again, so imagine that elastic pulling them together again.)
Swivel your right ankle now so it meets the left ankle again.
This is basic Shikko knee walking, the Samurai Walk."
Bokken - Art of the Japanese Sword (1986). David Lowry. Black Belt Books.
This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License.